Field of the Invention
The present invention relates to image forming apparatuses, such as a copier and a laser printer, and in particular to printing mode switching control for double-sided printing in a color image forming apparatus.
Description of the Related Art
Conventionally, an electrophotographic color image forming apparatus has a mode capable of multi-color image forming (e.g., full color mode) and a mode capable of single color image forming (e.g., black and white mode). The two modes are configured to be switchable. Switching the mode every time when a user designates a color image forming apparatus to print a job mixedly including a full color image and a black and white image reduces productivity owing to downtime for mode switching. If all images including black and white images are printed in the full color mode, the operating lives of consumable items other than of black are unnecessarily reduced. Thus, an image forming apparatus has been proposed that optimizes mode switching timing according to the total number of sheets for a job and the number of black and white image sheets included in the job.
For example, Japanese Patent Application Laid-Open No. 2008-116906 discloses a method of determining mode switching timing based on the total number of pages for a job, the number of printed pages, and a mode switching threshold stored in an image forming apparatus, for processing a job mixedly including a black and white image and a full color image. According to Japanese Patent Application Laid-Open No. 2008-116906, the image forming apparatus is switched from the full color mode to the black and white mode if the number of remaining pages for a job exceeds the switching threshold, and printing is performed, thereby reducing the number of times for switching to the black and white mode and optimizing the total print time period. Likewise, Japanese Patent Application Laid-Open No. 2003-262999 also discloses a method of determining mode switching timing for a job mixedly including a black and white image and a full color image. According to Japanese Patent Application Laid-Open No. 2003-262999, mode switching from the full color mode to the black and white mode is executed if images of a job designated by a user indicate that a predetermined number of black and white images are consecutively arranged, and the black and white images are formed. On the contrary, if the number of black and white images is less than the number of predetermined sheets, the black and white images are formed, with the mode being left in the full color mode without mode switching. Thus, the frequency of execution of mode switching is optimized. That is, the productivity and the operating lives of consumable items are optimized.
In recent years, in view of savings in resources and energy, image forming apparatuses standardly having an automatic double-sided printing function have increasingly been installed. Meanwhile, both requirements of reduction in costs of image forming apparatuses and improvement in productivity of double-sided printing have been required to be met. For example, to achieve reduction in cost, there is a configuration that includes a driving source for a reverse mechanism embedded in an image forming apparatus to revert the front and back of a recording sheet, the driving source being shared with a fixing device, and omits a clutch for transmitting a driving force to the reverse mechanism. If this configuration executes a job of double-sided printing mixedly including a black and white image and a full color image, the mode cannot be switched according to an order in some cases. That is, situations occur where a recording sheet cannot be temporarily under stand-by in the reverse mechanism due to mode switching. To temporarily cause a recording sheet to stand by in the reverse mechanism, all operations related to the fixing device are also required to be temporarily stopped. Typically, returning operations related to the fixing device require a certain period after transmission of a driving force. Consequently, during the returning operations, the driving force is transmitted also to the reverse mechanism that shears the same driving source with the fixing device because the clutch is omitted. The recording sheet standing by in the reverse mechanism is thus conveyed to the reverse path, and collides with another recording sheet standing by on the reverse path, thereby causing printing failure.
If the image forming apparatus having the above configuration executes a job mixedly including a black and white image and a full color image entirely in the full color mode, no mode switching is required. Consequently, no printing failure occurs. However, if image forming is continued in the full color mode even for the black and white image, the operating lives of consumable items that are other than items for black and are not originally used for this image forming are reduced. This reduction is unfavorable for the user. It is thus an object to allow the image forming apparatus including the aforementioned cost-reduced reverse mechanism to execute mode switching at an appropriate timing in execution of a job mixedly including the black and white mode and the full color mode, without causing printing failure.